Problem: The scale on a map is 10cm : 7km. If two cities are 30cm apart on the map, what is the actual distance in km between the two cities?
Answer: The scale means that every 10cm on the map corresponds to an actual distance of 7km. A distance of 30cm on the map is the same as 3 $\cdot$ 10cm. The actual distance between the two cities is 3 $\cdot$ 7km, or 21km.